The invention relates to the general field of photolithography with particular reference to formation of ant-reflective coatings for use with deep sub-micron technology.
Ant-reflective layers (AFLs) are now widely used at interfaces during photo-lithographic processing of photoresists. Among the problems that may occur if no ARL is present we include reflective notching (associated with variations in surface topography), standing waves that develop as a result of interference between incoming and reflected waves, and back-scattered light in general. These problems are particularly severe in the deep UV (248 nm) range of exposing wavelengths because of difficulties in controlling the critical dimension.
The ARL is laid down over the surface that is to be patterned just prior to the laying down of the photoresist. ARL materials may be either organic or inorganic. Organic ARLs, such as pigments, have the advantage of being easy to deposit uniformly and, additionally, have good planarization, making for a constant photo-resist thickness above them. They do, however, also have the significant disadvantage of having a fixed refractive index and extinction coefficient as well as poor thickness control. This means that their optical thickness cannot be adjusted for maximum effectiveness relative to a given layer of photoresist. They may also be difficult to remove after photoresist stripping.
The optical characteristics of inorganic ARLs are subject to easy control and they can be readily removed using standard etch processes. On the other hand, they have a conformal topography so the thickness of the photoresist relative to the ARL will not be constant.
In weighing the conflicting properties of these two types of ARL, the inorganic variety have been our first choice because they can be deposited through CVD (chemical vapor deposition) which is a mature technology. Their refractive index n and extinction coefficient k can be tailored to be optimum after the lithography details have been determined through simulation. Silicon oxynitride has been among the most popular of the inorganic ARLs because it is easy to deposit by CVD, specifically plasma enhanced (PE) CVD, and its precise composition can be adjusted to obtain the desired n and k values
The oxynitride based ARLs do, however, suffer from a problem of their own: A line formed on substrate 11 in photoresist through standard photolithographic processing should have the profile shown as 15 in FIG. 1. When, however, ARL 21 of the SiON type is inserted between the resist and the substrate, the photoresist line profile may develop a footing 23, as shown in FIG. 2b, or an overhang 22, as shown in FIG. 2a. This comes about because of the presence of amino groups in the ARL. Diffusion of acid towards the substrate can result in the formation of a footing while acid evaporation from the resist""s top skin (during post-exposure delay) can result in the formation of an overhang.
The present invention teaches how these problems may be avoided by using an inorganic ARL that is free of nitrogen. The method for preparing the ARL is less subject to contamination than processes that are currently described in the prior art.
A routine search of the prior art was performed. The following references of interest were found:
In U.S. Pat. No. 5,968,324, Cheung et al. disclose an ARL made using silane and N2O while Cheung et al. (U.S. Pat. No. 6,083,852) show an ARL comprised of Si, N and, optionally, O. In U.S. Pat. No. 5,990,002, Niroomand et al. disclose another ARL process in which alternating layers of polysilicon and silicon nitride are used.
Forbes et al., (U.S. Pat. No. 5,926,740) show formation of ARLs that include silicon oxycarbide (SiOC). For example, a substrate is heated to a temperature of approximately 250 degrees Celsius. Silane (SiH4) and methane (CH4) gases are introduced in the presence of an RF (13.56 MHz) plasma at a power between 10 Watts and 100 Watts. No oxygen is present so only silicon carbide gets formed. According to another aspect of this invention, a second layer that includes SiOC is formed. The SiOC second layer is formed by high temperature pyrolysis of silicone polymer resins such as methyl trichlorosilane or dimethyl dichlorosilane. Use of these precursor materials does, however, introduces the possibility of the SiOC being contaminated by chloride ions.
It has been an object of the present invention to provide a process for forming an anti-reflective layer.
Another object of the invention has been that said anti-reflective layer not bring about the formation of footings or overhangs in photoresist patterns formed over it.
A further object has been that said layer be free of chlorine contaminants.
These objects have been achieved by means of a deposition process for silicon oxycarbide films based on plasma enhanced CVD of silane mixed with methyl-silane, trimethyl-silane, or tetramethyl-silane (together with a carrier gas). Provided the relative gas flow rates are maintained within the ranges disclosed in the present invention, films having excellent ARL properties are obtained, with photoresist patterns formed on said films being free of overhangs and footings.